1. Field of Invention
The invention relates to the identification and characterization of the SPG4 gene encoding spastin, which is responsible for the most common form of autosomal dominant hereditary spastic paraplegia (HSP), to the cloning and characterization of its cDNA, and also to the corresponding polypeptides. The invention also relates to vectors, to transformed cells and to transgenic animals, and also to diagnostic methods and kits and to methods for selecting a chemical or biochemical compound capable of interacting directly or indirectly with a polypeptide according to the invention.
2. Background of the Invention
Hereditary spastic paraplegias (HSPs) are degenerative disorders of the central nervous system, characterized by bilateral and progessive spasticity of the lower limbs. They reveal themselves clinically through difficulties in walking possibly evolving into total paralysis of both legs. The physiopathology of this set of diseases is, to date, relatively undocumented; however, anatomopathological data make it possible to conclude that the attack is limited to the pyramidal tracts responsible for voluntary motricity in the spinal cord (Reid, 1997). Various clinical and genetic forms of HSP exist. The so-called “pure” HSPs, which correspond to isolated spasticity of the lower limbs, are clinically distinguished from the “complex” HSPs, for which the spasticity of the legs is associated with other clinical signs of neurological or non-neurological type (Bruyn et al., 1991). From a genetic point of view, the HSPs can be transmitted according to the autosomal dominant (AD-HSP), autosomal recessive (AR-HSP) or X-linked (X-HSP) mode. The “pure” form of HSP, which is most commonly transmitted according to the autosomal dominant mode, remains the most frequent (approximately 80% of HSPs) (Reid, 1997). The incidence of HSPs, which remains difficult to estimate because of rare epidemiological studies and the considerable clinical variability, varies from 0.9:100 000 in Denmark, 3 to 9.6:100 000 in certain regions of Spain (Polo et al., 1991) or 14:100 000 in Norway (Skre, 1974) (approximately 3:100 000 in France).
In addition to this great clinical variability, which is observed not only between various families but also between various affected members of the same family, the HSPs are also characterized by considerable genetic heterogeneity. In the case of AD-HSPs, four loci have been identified, to date, on chromosomes 14 (locus SPG3) (Hazan et al., 1993), 2 (locus SPG4) (Hazan et al., 1994; Hentali et al., 1994), 15 (locus SPG6) (Fink et al., 1995) and 8 (locus SPG8) (Hedera et al., 1999). The study of a large number of families exhibiting an AD-HSP has shown that the gene carried by chromosome 2 is a main locus of this form of the disease, found in 40 to 50% of the families analyzed (The Hereditary Spastic Paraplegia Working Group, 1996; Durr et al., 1996). An anticipation phenomenon was observed in some locus SPG4-linked HSP families; this phenomenon has, subsequently, been associated with the expansion of a (CAG)n repeat demonstrated in 6 Danish families (Nielsen et al., 1997) using the RED (for Rapid Expansion Detection) technique. It has, however, never been possible to confirm this expansion in any of the families tested by this method or by the systematic search for sequences of (CAG)n type in physical maps composed of YAC (for Yeast Artificial Chromosome) or BAC (for Bacterial Artificial Chromosome) clones (Hazan et al., Genomics, 60 (3), 309-19, 1999).
To date, three genes responsible for two forms of X-HSP and one form of AR-HSP have been identified. Mutations in the gene which encodes a neuron-specific cell adhesion molecule, L1-CAM (for L1 Cell Adhesion Molecule), and which is located at Xq28 (locus SPG1) cause a complex form of HSP (Jouet et al., 1994) in which the spasticity is associated with a mental handicap, whereas mutations in the PLP (for ProteoLipid Protein) gene located at Xq21 (locus SPG2), which encodes a constitutive molecule of the myelin layer, cause pure and complex forms of X-HSP (Saugier-Veber, P. et al., 1994). More recently, mutations in the gene located at 16q24.3 (locus SPG7), which encodes paraplegin, a mitochondrial ATPase of the AAA (for “ATPases Associated with diverse cellular Activities”) protein family (Confalonieri et al., 1995), have been associated with complex and pure forms of AR-HSP (Casari et al., 1998).
Thus, there remains, today, a great need to identify and characterize the gene responsible for the most common form of AD-HSP. The identification of this gene should, in particular, allow, besides the possibility of a test for antenatal screening in the families concerned, a better understanding of some of the molecular mechanisms engendering these degenerations specific for nerve bundles of the spinal cord, or even make it possible to provide an elementary response regarding therapeutic treatment for the patients.